DESCRIPTION (Applicant's Description) Colorectal cancer (CRC) is the second leading cause of cancer mortality and the fourth most common cancer in men and women in this country. CRC is usually preceded by adenomatous polyps. Colonoscopic removal of adenornatous polyps in the National Polyp Study (NPS) resulted in a lower than expected incidence of colorectal cancer. Screening colonoscopy has been calculated to be as cost effective for CRC mortality reduction as screening with fecal occult blood testing and sigmoidoscopy. Colonoscopy allows both identification and removal of premalignant polyps in a single examination, is now widely available, has low complication rates, and is becoming less costly. Whether colonscopy could be used as a once in a lifetime screening test to reduce the incidence and mortality of colorectal cancer would be of considerable importance. The goal of this proposed randomized controlled trial (RCT) is to determine the feasibility of a larger trial of once in a lifetime screening colonoscopy in an average-risk population of men and women ages 50-64. The larger RCT will determine whether a single screening colonoscopy can significantly reduce colorectal cancer mortality, primarily by reducing its incidence, and to estimate the magnitude and duration of its effect. Feasibility in the proposed studies will be assessed by the ability of the centers to enroll subjects in the trial, the ability to perform colonoscopy on all subjects, and the rate of finding cancer and adenomatous polyps with advanced pathology. The study will also provide data on the proportion of people in an asymptomatic, average risk population who have prevalent colorectal cancer detected by screening who require surgery, and the proportion with adenomas detected who require surveillance examinations. The prevalence and type of colorectal cancer screening in those in the usual care group will be evaluated. We will determine feasibility both in a managed care setting and in the private sector through community recruitment. More precise planning of the larger randomized larger trial of once in a lifetime screening colonoscopy can follow completion of the feasibility study.